Monitoring metabolic responses after induction of hypoxia in the KHT tumour using 31P NMR spectroscopy

31P MRS to monitor the induction of tumor hypoxia by the modification of the oxygen affinity of hemoglobin using BW 589C

PURPOSE

BW 589C induces severe tumor hypoxia by modifying the affinity of oxyhemoglobin, causing a left shift of the oxygen-hemoglobin dissociation curve. 31P magnetic resonance spectra (MRS) was used to monitor the effects of BW 589C on tumor energy metabolism in three experimental tumor models.

METHODS AND MATERIALS

HT-29 colon xenograft, murine transplantable RIF-1 fibrosarcoma and KHT sarcoma were studied in unanesthetised mice. 31P MR spectra were acquired on a 4.7 Tesla magnet before administering oral BW 589C (250 mg/kg) and after 3, 6, and 24 h. Samples of tail vein blood were then taken for 2,3 DPG levels for RIF-1 and HT-29 tumors.

RESULTS

Doubling of inorganic phosphorus (Pi) to total phosphorus was observed 5-6 h after BW 589C for all three tumor types. Although the left shift due to BW 589C persists at 24 h, the level of Pi to total phosphorus returned to baseline with no significant difference from control values for the RIF-1 and HT-29 tumors. These results suggest that there was cellular metabolic adaptation to the reduction of oxygen delivery by BW 589C. This does not appear to involve 2,3 DPG as there was no significant alteration in tumor levels. The death of hypoxic cells may, also, have contributed to the recovery of Pi to total phosphorus.

CONCLUSION

The efficacy of bioreductive drugs can be enhanced by increasing the severity of tumor hypoxia. 31P MRS in conjunction with other techniques for assessing the intratumor environment could play an important role in planning cancer therapy.

Assessing the bioreductive effectiveness of the nitroimidazole RSU1069 and its prodrug RB6145: with particular reference to in vivo methods of evaluation

The nitroimidazole, RSU1069, has been shown to have a very high differential toxicity towards hypoxic cells compared to oxic cells both in in vitro and in vivo experimental conditions. However, in the clinic it was found to cause severe emesis and had to be withdrawn. After an extensive drug development programme an analogue of RSU1069, RB6145, which acts as a pro-drug for RSU1069, was found to be the most suitable candidate for further investigation. In in vivo studies with murine tumour models, when RB6145 was used in combination with X-rays it was shown to produce a similar level of toxicity towards hypoxic cells as that observed for RSU1069. Its activity was the same whether it was administered interperitoneally or orally and the same level of anti-tumour effect was observed if the drug was given before or after X-rays. RB6145 is better tolerated systemically in mice than RSU1069 and canine studies have shown that it is less emetic than the parent drug. Bioreductive drugs can also be used in combination with treatments that preferentially increase tumour hypoxia. Photodynamic therapy (PDT) causes extensive vascular damage in tumours. If either RSU1069 or RB6145 are administered during PDT, very large increases in the growth delay induced by PDT alone are seen for the RIF-1 murine tumour. RB6145 has been accepted for clinical toxicity trials with the prospect of using it in combination with X-rays. In the future it may also be of clinical use with treatments such as PDT.